Carbon has three naturally occurring isotopes: 12C, 13C, and 14C. 12C and 13C are stable, while 14C is radioactive, hence the term "radiocarbon." 14C is a cosmogenically-produced isotope, forming through the bombardment of 14N atoms by cosmic radiation spallation products. 14C then binds with oxygen atoms in the atmosphere to form CO2. Plants uptake CO2 during photosynthesis, incorporating 12C, 13C, and 14C into its cells. When a plant or organisms dies, its carbon isotopes are locked in. Because 14C is radioactive, its abundance in an organic compound decreases over time, while the amounts of 12C and 13C remain constant. This is the basis of radiocarbon dating.

The abundance of 14C in a sample is measured by accelerator mass spectrometry (AMS). Using this highly sensitive measurement technique, we can measure the ratio of 14C to 12C in samples and of standard materials with a known 14C/12C ratio. 14C measurements are reported relative to a "modern" standard material (NBS Oxalic Acid 1, "OX-1", NIST-SRM-4990). "Modern" is this case is defined as 95% of the 14C activity of the OX-1 standard in the year 1950. Nuclear weapons testing in the late 1950s to 1960s produced excess 14C in the atmosphere, an abrupt increase we call the "bomb spike." Because of changes in atmospheric 14C content over time, radiocarbon ages must be calibrated using a 14C calibration curve. This can be done using open-source tools like OxCal.

Rather than calculating a radiocarbon age, we can also express 14C activity as "Fraction Modern" or "F14C." SAmples with F14C = 1 are considered "modern," and have the same 14C activity as the OX-1 standard material. Samples with a F14C > 1 have incorporated "bomb 14C" and likely have a rapid turnover time. This is common for modern vegetation in the northern hemisphere. Conversely, organic-rich rocks, like marine black shales formed millions of years ago, are "radiocarbon dead," with F14C = 0. 

In Earth Surface Geochemistry, radiocarbon is useful as a carbon tracer. By measuring the carbon isotope composition of soil and sediment, we can infer how long carbon can persist at Earth's surface, how much carbon is mobilized through the erosion of old soils and shale bedrock, and what happens to carbon as it is transported through river systems.

Soil and sediment can comprise a mixture of carbon molecules that come from different plant and organic matter sources and range in age. We can use 14C measurements to determine how long carbon has accumulated in soil and sediment. Inversely, these measurements of 14C in soil and sediment can also be used to determine how fast or slow organic carbon decomposes at Earth's surface. The organic matter "turnover rate" is roughly equal to the inverse of the oldest 14C-age measured in a soil or sediment sample.

References cited above:

Schuur, E. A., Druffel, E. R., & Trumbore, S. E. (2016). Radiocarbon and climate change. Switzerland: Springer International Publishing Switzerland. https://link.springer.com/book/10.1007/978-3-319-25643-6

Torn, M. S., Trumbore, S. E., Chadwick, O. A., Vitousek, P. M., & Hendricks, D. M. (1997). Mineral control of soil organic carbon storage and turnover. Nature, 389(6647), 170-173.